In the field of optical fiber communications the need for maintaining the end of an optical fiber in fixed position relative to another optical fiber, a planar waveguide, an opto-electronic device, or some other part frequently arises. Typically the end of the optical fiber is to be maintained in optical coupling relationship with the other part.
A known way of maintaining the end of an optical fiber in a fixed predetermined position comprises providing a semiconductor substrate with a V-groove therein, placing the fiber into the V-groove, and maintaining the fiber in the groove by appropriate means. This approach is used, for instance, in an optical fiber connector disclosed in U.S. Pat. No. 3,864,018. See also C. M. Miller, The Bell System Technical Journal, Vol. 57(1), pp. 75-90 (January 1978), and C. M. Schroeder, ibid, pp. 91-97. A different approach for coupling an optical fiber to a channel waveguide is disclosed by Y. Yamada et al in Journal of Lightwave Technology, Vol. LT-5(12), pp. 1716-1720 (December 1987).
Semiconductor chips comprising accurately placed, shaped and dimensioned V-grooves can be readily produced by known photolithographic and etching techniques. See, for instance, U.S. Pat. Nos. 3,765,969 and 3,506,509, which disclose means for anisotropic etching of Si. See also K. E. Bean, IEEE Transactions on Electron Devices, Vol. ED-25(10), pp. 1185-1193 (October 1978), incorporated herein by reference, and K. E. Petersen, Proceedings of the IEEE, Vol. 70(5), pp. 420-457 (May 1982).
As is well known, optical fibers typically comprise one or more polymer coatings that protect and strengthen the fiber. Since the thickness of the coating typically is not controlled to the same degree that the thickness and concentricity of the fiber are controlled (as well as for other reasons), the coating is generally stripped from the end portion of the fiber before the fiber is coupled to another fiber or device. A bare fiber is, however, relatively fragile, and care must be taken to insure that the bare fiber end is maintained substantially free of mechanical stress. This problem is alleviated if the semiconductor substrate contains a tandem V-groove, i.e., a V-groove that can accommodate a short length of bare fiber together with a portion of the coated fiber. In such a tandem V-groove the coated fiber portion can be epoxied into (or otherwise affixed to) the large cross-section portion of the V-groove, thereby providing strain relief, whereas the bare portion of the fiber is affixed to the small cross-section portion of the V-groove in the usual manner.
The prior art method of making a tandem V-groove in (100) silicon comprises providing an appropriately patterned masking layer (e.g., SiO.sub.2 or silicon nitride) that comprises a corner compensation feature of the type shown in FIG. 1. The method also comprises simultaneously etching both parts of the V-groove. See, for instance, K. E. Bean, op. cit. The corner compensation feature serves to limit undercutting in the transition region of the tandem V-groove, such that a relatively controlled transition between the large and small cross-section portions of the tandem groove results.
In FIG. 1 is schematically shown a portion of an article 1 comprising a tandem V-groove in (100) Si substrate 10 produced by the prior art method, wherein numerals 11 and 15 refer to the sloping side walls of the wide (14) and narrow (16) portions of the groove, respectively, 12 refers to the masked portion of the Si surface, and dotted lines 13 indicate the shape of the transition region between 14 and 16. Numerals 17 refer to the corner compensation feature of the masking layer.
The prior art technique has several shortcomings. For instance, the presence of the compensation feature restricts the flow of etchant which, in turn, may result in asymmetry of the groove. Furthermore, the small cross-section portion of the tandem groove is etched to completion long before etching of the remainder of the groove is completed. This may result in undercutting of the former, with attendant reduction of dimensional control. Still further, the "ears" (17) resulting from the compensation feature may be in the way of the fiber during placement of the fiber into the groove, since it may not always be practical to remove the ears by etching or other appropriate procedure. Finally, certain combinations of feature sizes may be completely unattainable by means of corner compensation.
In view of the advantages that a tandem V-groove can offer, a method of producing such a groove that is not subject to some or all of the shortcomings of the prior art method would be of interest. This application discloses such a method.